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CHAPTER 6. FLOW, BEDFORMS AND STRATIFICATION UNDER OSCILLATORYAND COMBINED FLOWS

INTRODUCTION

Bedforms and stratification produced by unidirectional flows are well understood after almost a century ofstudy in flumes and natural settings. However, in the geologic record of marine and lacustrine depositionalenvironments other classes of fluid flow are important in producing structures that are preserved and form aninterpretive basis for those deposits. Figure 6-1 summarizes a continuum of flow types, from purely unidirectionalflows to purely oscillatory flows, between which are a range of mixed flows and their components are shown in thelower half of the figure. In this section we will begin by examining the nature of currents, bedforms and stratificationproduced by wind-generated waves on a free water surface (i.e., purely oscillatory flow). This will be followed bya very brief description of the bedforms and stratification that develop when unidirectional and oscillatory currentsare superimposed to produce a general class of flow termed a combined flow. Note that figure 6-1 is a simplificationof possible natural flows. To begin with, the classification shown in figure 6-1 does not consider the angularrelationship between the oscillatory and unidirectional components: the resultant current produced by co-linearflows (i.e., both act in the same direction) will be much less complex than in the case where the oscillatory componentacts at some angle to the unidirectional component. In addition, in natural flows several different oscillatory andunidirectional components may act simultaneously to result in a very complex flow pattern. In nature such patternsexist and experimental work, like that described in the section on bedforms, is limited to the relatively simplisticapproach.

GENERAL CHARACTERISTICS OF GRAVITY WAVES

Waves generated by wind blowing over a water surface are prevalent in most marine and lacustrine settings.Such waves are commonly referred to as gravity waves because gravity is the most important force acting to dampenthese waves (i.e., gravity acts to restore the water surface to a flat surface after a wave has been generated by wind).

Figure 6-1. Flow classes defined in terms of the relative strength of their oscillatory and unidirectional components. Top graphsillustrate the pattern of variation of velocity over time of the net flow and the lower graphs show the oscillatory and unidirectionalcomponents of each flow separately. After Swift et al., 1983.

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Resultant Flow

Flow components

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Time (seconds)

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Gravity waves are just one particular type of wave that acts in large water bodies such as oceans (Fig. 6-2) and theyaccount for much of the total energy possessed by the worlds oceans.

The majority of gravity waves on a water surface are generated by wind moving over that surface. Whilethe exact mechanisms of wave formation are not well understood the response of the water surface to wind is wellknown. Waves develop as sinusoidal oscillations of the water surface that propagate (i.e., travel) in the directionof the wind. The characteristics of wind-generated gravity waves (e.g., their size) depends on a number of factorsthat include wind speed, duration of wind and the distance over which the wind acts on the water surface (thisdistance is termed fetch). Waves will propagate beyond the region of wave generation and may travel thousandsof kilometres and take days to dissipate. Waves that have left their region of generation are termed swell. Swellwaves are typically very long, relatively low and straight-crested.

Any description of gravity waves must include wave length (L; also referred to as wave spacing), waveheight (H), wave celerity (C; the speed at which the wave moves along the water surface, and wave period (T, whereT = L/C; the time required for one full wave-length to pass a fixed point). All of these wave characteristics aredetermined by the intensity, duration and fetch (the distance over which the wind acts on a water surface) of thewind that generates them. To illustrate figure 6-3 shows the scales of waves produced by various wind speeds actingover seas with unlimited fetch.

Gravity waves are classified by the relationship between their wave length and the depth of water throughwhich they are moving (this affects the form of the waves and waves in deep water undergo a number of changesas they move into shallow water; see below). Deep water waves are waves with lengths that are no more than twotimes the depth (h) of water over which they are moving (i.e., h > L/2; note that this depth is commonly called wavebase). Transitional waves have longer wave lengths relative to water depth, between the limits L/20 < h < L/2. Shallowwater waves are waves have lengths that are at least 20 times the water depth (i.e., h < L/20).

The geologically important result of gravity waves on the water surface is the fluid motion that they generate,motion that may result in sediment transport and the formation of primary sedimentary structures. The nature offluid motion associated with waves is very complex and this section will only discuss this topic in a very superficialmanner and in terms that will be particularly important to later discussion of the sedimentary structures generatedby waves. The best general text that describes gravity waves and their products is that by Komar (1976).

To begin the discussion of fluid motion under waves we will consider so-called deep water waves that donot interact significantly with the bottom (i.e., the sediment substrate at some depth below the water surface isunaffected by the passage of such waves). Figure 6-4A shows the characteristics of a train of deep water waves.Fluid motion beneath such waves is complex and is shown in a simple but instructive manner in Fig. 6-4A. If we

Figure 6-2. Wave classification by wave periodand the mechanisms that generate waves on largewater bodies such as oceans. Each shaded bar showsthe range of wave periods generated by each mecha-nism and the solid black bars indicate the waveperiod that generates the maximum wave energy inthe oceans due to each generating mechanism. Thebar at approximately 10 seconds represents wind-generated waves by average wind conditions on theoceans; the bar representing a period of just over100 seconds is associated with long period and wavelength “swell” generated by storms. The bar asso-ciated with wave periods of approximately 2000seconds (about half an hour) are waves generated byearthquakes (Tsunamis). The two bars on the farright represent, from left to right, tides withsemidaily (or semidiurnal; two tidal cycles per day)and daily (or diurnal; one tidal cycle per day)periods.

10.10.01 10 100 1,000 10,000 100,000 1,000,000

Wave Period (seconds)

wind-generated gravity waves

storms

diurnal semi-diurnalEarthquakes(tsunamis)

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ism

of

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follow the path of a single water molecule as a wave passes one complete wave-length the path appears circular:motion is in the direction of propagation under the crest and in the opposing direction under the trough and atintermediate angles and directions, through 360°, at other positions beneath the passing wave (Fig. 6-4A). Thiscircular path is referred to as the wave orbital and its diameter is termed the orbital diameter (do). Under deep waterwaves the orbital diameter becomes exponentially smaller with depth until it is of negligible size and the orbitaldiameter is related to depth by:

do He Ly

=2π

Eq. 6-1

where y is a negative number indicating the distance below the water surface and L is the wave length. The waveorbitals persist but become negligible below the depth equal to L/2. The speed of fluid motion about the orbitalsat any time during the passage of a single wave is termed the orbital speed. The orbital speed just as the crest andtrough pass by is termed the maximum orbital speed (Um); Um is equal but acts in opposite directions beneath thecrest and trough, as outlined above. The maximum orbital speed varies with depth by its relationship to the orbitaldiameter, given by:

Udo

Tm =

πEq. 6-2

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Figure 6-3. Wave conditions as a function of wind speed for a fully developed sea. Data from Thurman, 1988.

0Time

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ocity

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still water level

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do

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shal

lowin

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Figure 6-4. Characteristics of deep water and transitional waves. See text for discussion.

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At the water surface the orbital diameter is equal to the wave height (i.e., do = H), such that:

UH

Tm =

πEq. 6-3

As waves propagate into shallow water (at depths less than L/2) they begin to interact with the bottom andundergo several changes and the relationships given above no longer hold. A particularly important changeinvolves the refraction of the wave crests to a position that parallels the isobaths of the bottom (isobaths are linesjoining points of equal depth). Because isobaths approximately parallel the shoreline, refraction also aligns thewaves parallel to the shoreline. In addition, waves become steeper due to an increase in H and the formulae givenabove no longer accurately describe the behaviour of the current that they generate. Steepening increases untilthe deep water wave height becomes 75% of the water depth (i.e., h = 4H/3); at this depth the wave will break.Concurrent with steepening of the wave the wave orbitals become elliptical, and increasingly flatter with depth untilalong the bottom (Fig. 6-4B) the motion of fluid molecules (and any sediment in transport) follows a line that is parallelto the bottom surface. Under transitional waves the rate of decrease in orbital diameter with depth is much less thanthe rate under deep water waves (Fig. 6-4B). Under transitional waves the orbital diameter is given by:

dHT g

ho =

2πEq. 6-4

where g is the acceleration due to gravity. Note that under shallow water waves the orbital diameter does not changewith distance below the water surface but is constant. The maximum orbital speed under transitional waves is givenby the general relationship for shallow water waves :

UH

hghm =

2Eq. 6-5

(note that from here on we will assume that Um is the maximum orbital speed acting on the bottom because that iswhere the sedimentologically important work is done).

BEDFORMS AND STRATIFICATION UNDER PURELY OSCILLATORY CURRENTS

As under unidirectional flows, bedforms begin to develop under oscillatory flows as soon as the flowconditions exceed some threshold for the initiation of movement of sediment. Also, the forms of bedforms, andtheir associated internal stratification, vary with the strength of the current. However, because of the veryfundamental differences between unidirectional and oscillatory flows the properties of the flow that control whensediment will move and what bedform will be stable must also differ.

Initiation of sediment motion under waves

It is the to and fro fluid motion acting on the bottom that may produce bedforms and stratification if thestrength of the oscillatory current exceeds some threshold condition required for the initiation of particle motion.Under unidirectional flows we saw that the critical flow condition for motion of a particle depends on the size anddensity of the particle, the density of the fluid, and the boundary shear stress (that was related to the flow velocityand depth). The condition for the initiation of motion of sediment under oscillatory currents may be similarlydetermined by the fluid and sediment properties but the flow strength is normally represented by the maximum orbitalspeed and the orbital diameter or wave period. Orbital diameter and wave period are important components of flowstrength because they determine the spatial extent over which the maximum orbital speed acts and its duration.Komar and Miller (1973) determined that the critical condition for the movement of sediment under waves can bedefined in a manner that is superficially similar to Shield’s criterion for motion under unidirectional flows, by thegeneral relationship:

ρ

ρ ρ

U

gDC

d

D

nt

s

o2

( )−=FHGIKJ Eq. 6-6

where Ut is the threshold maximum orbital speed required to move sediment; ρ is the fluid density, ρs is the density

106

of the sediment, g is the acceleration due to gravity, and D is the size of the particles. C and n are constants thatare determined by the size of the sediment. For grain sizes finer than 0.5 mm C = 0.21 and n = 0.50. For grain sizescoarser than 0.5 mm C = 1.45 and n = 0.25. Note in Eq. 6-6 that do varies with wave period (see Eq. 6-2 and 6-4) sothat the threshold maximum orbital speed similarly varies with wave period: the longer the wave period the largerthe threshold maximum orbital speed required to move the sediment.

Bedforms under waves

For many years one particular type of bedform has been considered diagnostic of the influence of waveson a sediment substrate: symmetrical ripples (commonly called wave ripples and the modern term is 2-D vortexripples). Today we know that there are actually a variety of bedforms that develop under waves but beforeconsidering these in detail we will briefly focus on the common wave ripple.

So-called “wave ripples” are distinct from the “current ripples” produced by unidirectional flows by theirsymmetrical profile, relatively peaked crest and broad trough, and by their straight to bifurcating crestlines (Fig.6-5). These are a very common bedform in shallow marine sediments and occur extensively on bedding planeexposures. The spacing of such ripples (λ) ranges form centimetres to in excess of a metre and they range in heightfrom millimetres to about a decimetre. The overall size of wave ripples varies directly with grain size and small scalesymmetrical ripples form in fine sand and large scale symmetrical ripples form in gravel. When such ripples formthey are molded on the bed beneath the propagating gravity waves and their crests are aligned parallel to the crestsof the water surface waves (but do not confuse these with in-phase waves of unidirectional flows). An importantimplication of the alignment of ripple crests is that, like the wave crests, such ripples will be aligned parallel to regionalshoreline (a characteristic that was nicely demonstrated by Leckie and Krystinik, 1989). Some symmetrical rippleshave two crests, aligned at approximately 90° to each other, and normally one crest is better-developed (the dominantcrest) than the other. For many years such ripples were termed interference ripples and taken to represent thecondition when symmetrical ripples form under two sets of gravity waves that propagate at right angles to eachother. However, recent, as-yet unpublished, experimental studies have shown that interference patterns of ripplecrests can develop under certain conditions when only one wave train is active. Thus, the standard interpretation

SHORELINE

OSCILLATINGCURRENT

Figure 6-5. Schematic illustration of classical “wave ripples” or 2-D vortex ripples. Note the cross-sectional form and straightto gently bifurcating crestlines. The direction of oscillation and inferred shoreline orientation are shown for comparison. Notethat no scale is shown but ripple spacing may range from centimetres to in excess of a metre, increasing with grain size. Seetext for discussion.

107

of this particular form of symmetrical ripple is now in doubt.

Many basic textbooks that discuss the humble wave ripple point out that the spacing of such ripples isdetermined by the diameter of the circular wave orbital that forms them. In general, the ripple spacing is slightlyless than the orbital diameter acting on the sediment substrate (see Eq. 6-8). Considering the above discussion thisidea makes sense for transitional waves under water depths just less than L/2 when a more-or-less circular orbitalis present. However, it is difficult to conceive of shallow-water waves (as described above) producing such ripples.Figure 6-6 schematically illustrates observations of symmetrical ripple spacing (λ, the orthogonal distance betweencrests) from natural and experimental settings where the orbital diameters of the formative waves are known. Whenwe plot ripple spacing against orbital diameter the data occur in two indistinct groups. Some ripples have spacingsthat are directly related to wave orbital diameter by the relationship:

λ = 0 8. doEq. 6-7

Such wave ripples are termed orbital ripples. However, many wave ripples do not fall on the line defined by Eq.6-7 but deviate from that line because their wave-lengths are much shorter than the orbital diameter and such ripplesare termed anorbital ripples. Ripples with wave-lengths that fall between these two classes are termed suborbitalripples. Note that with increasing grain size the maximum ripple wave-length also increases (i.e., the coarser thesediment the larger the maximum possible ripple wave-length). This suggests that for any grain size there is an upperlimit to the size of ripples that will form. Below the limit, a single orbital ripple will exist for every orbital diameteracting on the sediment surface and beyond that limit several individual anorbital ripples will be stable under arelatively long orbital diameter. The relationship between ripples and wave orbitals is shown schematically in figure6-6.

The above discussion points to the fact that there are a variety of bedforms that develop under oscillatoryflows. Over the past decade, experimental studies have helped describe these bedforms in terms of their morphologyand behaviour and the hydraulic conditions that are necessary to form them. Harms et al. (1982), based largely onexperimental work in John Southard’s labs at M.I.T., described bedforms under oscillatory flows in the manner thatthe unidirectional bedforms are described. In the remainder of this section we will consider the sequence of bedformsthat develop under oscillatory flows much like we did the unidirectional flow bedforms in the previous chapter ofthese notes.

Consider an experiment where we induce oscillatory fluid motion (i.e., back and forth) over a sandy substratein a closed tunnel. There is no free-water surface but the speed of the oscillatory current varies gradually in both

Figure 6-6. Schematic illustration show-ing the relationship between symmetricalripple wave-length and the diameter ofwave orbitals. Data are not shown. Thedark, shaded line (labelled “orbital rip-ples”) is defined by ripples with lengthsequal to 80% of their associated waveorbitals. Solid lines (labelled with grainssizes) indicate the limiting ripple wave-length for the grain size indicated. Theshaded area represents the region ofanorbital ripples. The drawings illustratethe relationship between symmetrical rip-ples and their wave orbital for orbital rip-ples (upper left) and anorbital ripples (lowerright). See text for further discussion.

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Orbital diameter (do; cm)

108

speed and direction (over 180°) and the maximum velocity, in either direction, is comparable to the maximum orbitalvelocity under waves. The duration of the current in either direction is related to the wave period and the distanceover which the current moves during one oscillation is related to the wave orbital diameter. Like the mental experimentthat we conducted to describe bedforms under unidirectional flows, in this case we will consider the sequence ofbedforms that develops with increasing flow strength (maximum orbital speed) holding all other variables constant.Note that not all of the following bedforms will develop under a given wave period or bed grain size, as we shallsee when we look at a bedform stability diagram for oscillatory flows.

The sequence of bedforms that develops under oscillatory flows is shown schematically in figure 6-7. Justas the current begins to move the sediment (i.e., the maximum orbital speed exceeds the threshold for motion) thefirst bedforms that develop are termed rolling grain ripples. These bedforms are small with lengths less than 10cm and heights on the order of a few millimetres to approximately 1 centimetre. Rolling grain ripples are symmetricalin profile, have low slopes, and are straight-crested. Sediment movement over rolling grain ripples is as traction:grains roll back and forth under the oscillating current. This bedform is thought to be “metastable”, that is, overtime they slowly grow in size (particularly height) and become vortex ripples. Under somewhat larger maximumorbital velocities vortex ripples develop quickly and are stable. Vortex ripples include the common form of waveripple described above in the opening paragraphs of this section on bedforms. As noted above, they are symmetricalin cross-section and have slopes that are steeper than rolling grain ripples. Vortex ripple lengths very fromcentimetres in fine sand to in excess of a metre in gravel and similarly vary in height from approximately a centimetreto a decimetre. Sediment transport over vortex ripples is both in traction and suspension. The first vortex ripplesto develop are straight-crested (2-dimensional) as depicted in figure 6-5. However, as the orbital speed increasesvortex ripples become increasingly 3-dimensional in plan view. The 3-D vortex ripples grow larger (with wave-lengths in excess of 1 m) and form rounded bedforms with hummocks (areas of positive relief) and swales (areasof negative relief) on the bed (see top block diagram in Fig. 6-11). Such large, 3-D vortex ripples are sometimes referredto as “hummocky ripples”. Note that these large ripples may someday become thought of as dunes under oscillatoryflows. With a further increase in maximum orbital speed the 3D-vortex ripples become flatter and somewhat shorter

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REVERSING - CREST(long wave periods)

POSTVORTEX

VORTEX

ROLLING - GRAIN

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Figure 6-7. Schematic illustration of the bedforms that develop under oscillatory currents. From the bottom upwards thesequence of bedforms reflects the sequence that develops with increasing maximum orbital velocity (except for the reversingcrest ripples). See text for further discussion. After Harms et al. (1982).

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to form post-vortex ripples. These bedforms can be thought of as transitional forms with flat bed, an essentiallyflat, featureless bedform that develops under the highest orbital velocities and experiences intense sedimenttransport. The so-called flat bed of oscillatory flows is morphologically similar to upper plane bed of unidirectionalflows (and many authors term the oscillatory bedform plane bed). Another bedform that develops under oscillatorycurrents, but only under waves with relatively long periods, is termed reversing-crest ripples. These bedformsare generally less than 10 cm long and rather low and are somewhat asymmetrical. They derive their name from thefact that they reverse in direction as the current reverses. This characteristic is possible because under waves withlong periods the duration of flow in one direction is sufficiently long to generate a small, asymmetrical ripples (likeripples under unidirectional flows). Thus with each oscillation the current reverses and so does the direction ofmigration of the asymmetric ripples. These particular bedforms illustrate that bedforms under oscillatory flows arevery similar to those under unidirectional flows but the current acting in one direction, under normally short periodwaves, does not persist long enough to generate bedforms like those under unidirectional flows.

Figure 6-8 shows a bedform stability diagram for fine, quartz sand (0.15 to 0.21 mm) in terms of orbital speedand wave period (we are ignoring the effects of temperature). Note that diagrams for other grain sizes will havesimilarly formed fields but at different positions (i.e., for coarse sand the fields shift upwards so that all transitionsoccur at higher maximum orbital velocities). Figure 6-8 shows that the sequence of bedforms with increasingmaximum orbital speed are essentially as outlined above and reversing-crest ripples appear to replace otherbedforms at significantly high wave periods. The range of conditions over which post-vortex ripples are stableextends to lower maximum orbital velocities with increasing period. Figure 6-8 is after Harms et al. (1982) but morerecent experiments by Southard et al. (1990) have documented, in more detail, the development of vortex rippleswith increasing orbital speed.

Stratification formed by oscillatory currents

Like bedforms that develop under unidirectional flows, bedforms under oscillatory currents result in a varietyof forms of cross-stratification that differ in detail due to the geometry and behaviour of the generative bedforms.

1.0

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PERIOD T (sec)

MA

XIM

UM

OR

BIT

AL

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EE

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ec)

FLAT BED(sand movement)

POSTVORTEX RIPPLES

VORTEX RIPPLES( start as rolling - grain ripples )

THREE - DIMENSIONAL ( in part ? )

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ROLLING - GRAIN RIPPLE (metastable)

NO RIPPLE ACTIVITY

REVERSINGCREST

RIPPLES

??

??

??

??

Figure 6-8. Diagram illustrating the fields of stability of wave-formed bedforms on beds of fine sand (0.15 to 0.21 mm). AfterHarms et al. (1982).

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To make a broad generalization: the features of wave-formed cross-strata that distinguish them from those formedby unidirectional flows are (1) a wide variation in cross-strata dip directions (although this is not always the case;see below) and (2) the symmetrical, curved bounding surfaces that are sometimes preserved, reflecting thesymmetrical form of the bedform. In addition, as we saw in the section on grain fabric, particle alignment in thedeposits of wave-generated currents is distinctly different from that under unidirectional flows; under purelyoscillatory flows the vector mean dip of particles may be horizontal.

Figure 6-9 depicts the forms of cross-stratification that develop under waves with increasing maximum orbitalvelocity (i.e., under the various bedforms described in the previous section) under conditions of vertical aggradationof a bed. Note that exact forms of stratification will vary, depending on the aggradation rate and the rate of ripplemigration (if any; see below). This description follows that of Harms et al. (1982) and remains somewhat speculativealthough the discussion below includes some observations from recent experiments by Southard et al. (1990) andArnott and Southard (1990).

Rolling grain ripples produce thin, sub-parallel laminae that may or may not display internal cross-lamination.As the bedforms grow to 2-D vortex ripples the internal cross-laminae become prevalent and the form of cross-stratification is complex. Under purely oscillating currents the internal cross-strata produced by 2-D vortex ripplesform sets of alternately dipping laminae bounded by curved erosional surfaces. Such sets are sometimes said tobe “braided” and form a particular type of cross-stratification that is termed chevron cross-stratification. The exactform of the cross-stratification produced by 2-D vortex ripples will depend on the behaviour of the bedform, whichin turn depends on the symmetry of the oscillatory current. The thickness of cross-strata sets depends on the sizeof the ripples that form them; therefore, set thickness will be strongly influenced by the grain size of the sediment(larger bedforms and cross-strata sets are possible in sediment of coarse grain size). The form of cross-stratification

Flat bedNear - horizontal fine laminae

Three - dimensional vortex ripples( flatter)λ dm - m, I 15 - 20Swaley sets of cross laminae withset contacts sloping less than 10˚,hummocks rarely preserved

Three - dimensional vortex ripples( steeper)λ dm - m, I~10Hummocky sets of cross laminae with set contacts sloping as muchas 10 - 15˚

Two - dimensional vortex ripplesλ cm - dm, I= 6 - 10 Small intricately "braided" setsof cross laminae

Rolling grain ripplesλ cm, I largeThin laminae, low angle crosslamination or flat lamination

INC

RE

AS

ING

OR

BIT

AL

VE

LOC

ITY

Figure 6-9. Forms of cross-stratification produced by wave-generated bedforms on slowly aggrading beds. Note that “I” isthe ripple index (ratio of length to height). After Harms et al. (1982).

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shown in figure 6-9 is limited to purely oscillatory flows but in nature it is not unusual for even wave-generatedcurrents in shallow water to have an asymmetry that induces a somewhat stronger current in one direction. Suchasymmetrical currents may cause 2-D vortex ripples to migrate in the direction of the strongest component of theoscillatory current. Like current ripples, the form of cross-stratification that is preserved will vary with the rateaggradation of the bed. Thus, there are a variety of forms of cross-stratification produced by 2-D vortex ripplesthat depend on the combined effects of bed aggradation and the rate of ripple migration. Figure 6-10 shows thecontinuum of forms of cross-stratification that may be produced by 2-D vortex ripples as a function of bedaggradation and ripple migration rates. Note that with ripple migration and no bed aggradation the internal cross-strata dip only in one direction and might appear like cross-strata produced by current ripples under unidirectionalflows. The similarities continue for the migrating ripples as increasing aggradation rates produce climbing formsof ripple cross-stratification. The preservation of symmetrical ripple forms is probably necessary to allow reliabledistinction of cross-stratification produced by waves from current ripple cross-stratification formed under similarconditions of bed aggradation. In addition, even the migrating wave-generated bedforms may preserve local setsof cross-strata that dip in the direction opposing the average direction of ripple migration, the presence of suchsets should suggest wave-generated currents. Only when the ripples do not migrate in a preferred direction willtrue chevron cross-stratification develop in which the proportion of cross-strata dipping in one direction or the otherare approximately equal.

Returning to figure 6-9, with increasing maximum orbital velocity, as the bedforms grow and become morerounded and three-dimensional, the forms of cross-stratification change to mimic the morphology of the bedforms.As depicted in figure 6-9 there are two forms of stratification produced by 3-D vortex ripples. The first to develop,

NOMIGRATION,

NOAGGRADATION

AGGRADATIONRATE

aggradation, no migration

migration + aggradationlarge angle of climb

migration + aggradationsmall angle of climb

MIGRATIONRATE

migration, no aggradation

Figure 6-10. Schematic illustration showing the idealized forms of cross-stratification produced by 2-D vortex ripples as afunction of their rate of migration and the rate of bed aggradation. After Harms et al. (1982).

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with increasing orbital velocity above that which 2-D vortex ripples are stable, is characterized by sets of bothconcave- and convex-up internal strata (termed swaley strata and hummocky strata, respectively) bounded bysimilarly shaped bounding surfaces. This form of cross-stratification is termed hummocky cross-stratification(HCS), consistently one of the most enigmatic primary sedimentary structures over the past two decades (see thefinal section of this chapter for a closer look at this form of cross-stratification and the debate that it has generated).Figure 6-11 shows a large, hummocky, 3-D vortex rippled bed surface and the form of HCS that the bedform is thoughtto produce. Internal lamination in HCS dip at angles up to 15° and are isotropic (i.e., dip with equal frequency inall directions). Based on the descriptions of the bedforms that are believed to generate HCS, the spacing ofhummocks should vary from approximately a decimetre to in excess of a metre. With increasing maximum orbitalvelocity figure 6-9 suggests that the next form of stratification is similar to HCS except for two distinctivecharacteristics: internal laminae dip at shallower angles (not exceeding 10°) and only swaley (concave) laminae arepreserved. This form of cross-stratification is termed swaley cross-stratification (SCS). The different form of SCS,compared to HCS, may be attributed to the lower relief of the bedforms and the greater scour of the bed (planingoff hummocks) under the greater maximum orbital velocities. With increasing maximum orbital velocity, throughthe field of post-vortex ripple stability, the internal strata and their bounding surfaces must become increasinglyflat and produce a form of horizontal lamination by deposition on a wave-generated flat bed. This form of horizontallamination has not been extensively studied and descriptions of such lamination does not provide a basis fordistinguishing it from upper plane bed horizontal lamination of unidirectional flows. Intuition suggests that internalfabric may provide a diagnostic criterion for the identification the formative processes responsible for this structure.Note that the internal structure of reversing crest-ripples is not well known. Presumably, with bed aggradation, theywill form laterally and vertically alternating (or at least varying) sets of cross-strata that dip in opposing directions.

Figure 6-11. Schematic illustrations of hummocky cross-stratification. A. A block diagram showing the form of thethree-dimensional bedform that is thought to produce HCS (3-D vortex ripples). B. The details of the internal structureof HCS. See text for details. From Cheel and Leckie (1992).

First-order surface

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Sole marks

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Figure 6-12. Plan view of ripples produced on beds of 0.28 mm sand by current, waves and combined flows. Note that netflow is from right to left and the boxes are 75 cm across. After Harms (1969).

The asymmetrical ripple form, if preserved, should distinguish this form of such cross-stratification from thatproduced by 2-D vortex ripples.

BEDFORMS AND STRATIFICATION UNDER COMBINED FLOWS

This brief section relies heavily on the results of recent work on the bedforms and stratification that developunder combined oscillatory and unidirectional flows. The flows considered are the result of the superposition ofwave-generated currents (see above) on simple unidirectional currents to produce temporal variation in flowstrength (e.g., velocity) as shown for combined flows in figure 6-1. Because this section is brief, the descriptionof both bedforms and their stratification will be combined.

Figure 6-12 shows the differences in plan form of wave ripples (2-D vortex ripples; formed under purelyoscillatory flows), current ripples (produced under purely unidirectional flows) and combined flow ripples. Notethat the major differences are the smaller spacing, straighter crests of wave ripples, and symmetrical profile of waveripples, compared to combined flow and current ripples, and the changes are gradual from one ripple type to theother. Internal stratification may reflect the differences in ripple behaviour: current ripples may have betterdeveloped unimodal dip directions of internal cross-strata. However, as noted above, true wave ripples may alsomigrate in one direction to produce similarly unimodal-dipping internal cross-strata.

Figure 6-13 is based on experimental work by Arnott and Southard (1989) who used a combined flow waveduct to simulate waves with an 8.5 second period acting on a bed of very fine sand. The apparatus was similar tothat used to simulate purely oscillatory flows but included a recirculating pump to induce a unidirectional currentwithin the duct. The graph shown in figure 6-13 is one of many such graphs that could be constructed for combinedflow, each representing a narrow range of grain size and wave periods. The sequence of bedforms along the verticalaxis is the sequence described for purely oscillatory flows (note that they did not go to high enough maximum orbitalvelocities to produce a wave-formed flat bed). Note also, that this figure suggests that there is a gradual transitionfrom 2-D vortex ripples to 3-D vortex ripples and current ripples. The large 3-D vortex ripples have been termedhummocky ripples in an attempt to emphasize that these bedforms likely produce the hummocky cross-stratificationthat is common in the geologic record of shallow marine sediments. These hummocky ripples appear to be stable

CurrentRipples

current - dominated wave - dominated

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only under purely oscillatory flows or under combined flows with only a very weak unidirectional component. Whenthe velocity of the unidirectional component of a combined flow is only a few percent of the oscillatory velocitythe hummocky bedforms become asymmetric in profile and migrate in the direction of the unidirectional component,producing low angle inclined cross-strata that are isotropic (dip in one direction in contrast to the anisotropic dipof hummocky cross-strata). With a further increase in the strength of the unidirectional component the bedformsbecome steeper and more asymmetric and produce high angle cross-stratification. Note that these large bedforms,formed under combined flows are termed dunes in figure 6-13 (they were termed “large ripples” in the original paperby Arnott and Southard, 1989) and that they form under much lower unidirectional velocities than under purelyunidirectional flows because of the superimposed oscillatory current. Indeed, combined flows appear to producelarge, dune-like bedforms that would not develop under purely unidirectional flows over fine sand. In addition, thesuperposition of an oscillatory component reduces the threshold unidirectional flow strength required to produceupper plane bed (or flat bed).

Figure 6-14 is similar to figure 6-13 but includes a wider range of orbital and unidirectional flow velocitiesand figure 6-15 schematically shows the forms of cross-stratification produced by the various bedforms. Figure6-15 indicates that there is a more-or-less gradual change in the styles of cross-stratification produced by oscillatoryand combined flows but that the overall geometry of the stratification should provide a basis for distinction of thegenerating flow types. However, this work is currently in its infancy and will require further experimentation andobservations from the ancient record before we have a good foundation for interpreting the formative processesthat produce these primary structures.

THE ENIGMA OF HUMMOCKY CROSS-STRATIFICATION

Note: Earlier in this chapter we introduced a form of cross-stratification termed hummocky cross-stratification (HCS) andattributed its formation to the presence of 3-D vortex ripples on an aggrading bed. This view of HCS is one of several that appearin the literature and the debate on the origin of this primary sedimentary structure is ongoing. This section aims to focus onHCS and to show that its interpretation is not so straight-forward.

Hummocky cross-stratification became popular during the late 1970’s and early 1980’s as its widespreadrecognition followed description by Harms et al. (1975), although it had been earlier reported under different names(e.g., truncated wave-ripple laminae, Campbell, 1966; crazy bedding, Howard, 1971; truncated megaripples, Howard,1972). The presence of HCS has since become a prime criterion for the recognition of ancient shallow-marine stormdeposits; however, its reliability as an unequivocal criterion for this environment is now less certain. Despite thefact that HCS is widely accepted to be the product of waves, the structure continues to be the focus of ongoingdebate regarding its mode of formation and, by implication, its specific paleohydraulic interpretation. HCS in marinedeposits has been variously attributed to formation by oscillatory flows produced by waves, combined oscillatoryand unidirectional flows, and purely unidirectional flows. This diversity of hypotheses for HCS formation is justified

no movement?

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10 cm scale insketches Figure 6-13. Very schematic illustration show-

ing the bedforms and their internal stratifica-tion produced in a duct by combined flows withan oscillatory component with a period of 8.5seconds acting on a bed of very fine sand. Notethat structures along the vertical axis are formedunder purely oscillatory flows and structuresalong the horizontal axis are formed underpurely unidirectional flows. After Arnott andSouthard (1989) and Duke, Arnott and Cheel(1991).

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NoMovement

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Figure 6-14. Extended bedform stability diagram for combined flows with an 8.5 second period oscillatory component actingon a bed of fine sand. Compare with figure 6-15 for the forms of cross-stratification produced under the conditions shown.After Myrow and Southard (1991).

Figure 6-15. Highly schematicillustration showing the forms ofcross-stratification produced bycombined flows with an 8.5 sec-ond period oscillatory componentacting on a be dof fine sand. Com-pare with figure 6-15. Based onMyrow and Southard (1991).

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because the basis for its interpretation is not as sound as that for many other sedimentary structures that are readilyvisible in modern settings or produced in laboratories. For example, our knowledge of the relationship betweenpaleohydraulics and stratification formed under unidirectional flows is quite advanced because we can easilydissect recognizable bedforms developed under known hydraulic conditions in rivers, intertidal areas and flumes,to precisely document the geometry of their internal structure. In contrast, HCS is found largely in ancient sedimentsand sedimentary rocks where paleohydraulic conditions must be inferred from associated deposits and structures.Thus, HCS has been largely interpreted on the basis of inference rather than direct observation of the relationshipbetween hydraulic processes and the form of the structure. The argument over the origin of HCS has been furthercomplicated by the recognition of similar forms of stratification in settings in which wind-generated water surfacewaves were an unlikely mechanism in its formation. The growing range of physical settings in which HCS-likestratification may have formed may justify the suggestion (Allen and Pound, 1985) that HCS has become just a“bucket term” that embodies similar stratification styles that may be generated by a variety of processes orcombinations of processes.

This section of these notes aims to describe HCS, in detail, and indicate that there is no consensus at thistime as to the exact origin of this form of cross-stratification although recent data points to an origin of HCS underpurely or strongly oscillatory dominant flows.

HCS — description and associations

Because of the paucity of modern and experimental examples of HCS, its paleohydraulic interpretation haslargely been based on its preserved characteristics in the geological record. The following description will reviewthe basis for the recognition of HCS, including its common stratigraphic and sedimentologic associations, andstress characteristics that have led to the various ideas on HCS formation.

Characteristics of HCS

Grain Size

The grain size of sediment in which HCS occurs varies from coarse silt to fine sand (Dott and Bourgeois,1982; Brenchley, 1985; Swift et al., 1987). HCS in coarser sediment is relatively rare but has been reported. Brenchleyand Newall (1982) described HCS in sandstones with mean grain sizes ranging from 0.7 - 1.1 mm (i.e., up to coarsesand). Walker et al. (1983) noted that gravel may comprise beds displaying HCS. However, in cases where gravel-size sediment is present in HCS, the gravel normally makes up a small proportion of the total grain size distributionand is found largely as lag-deposits on surfaces within or at the base of beds displaying HCS. Therefore, theconsensus seems to be that HCS is most common in very fine to fine-grained sand with the frequency of itsoccurrence decreasing dramatically with increasing grain size.

Morphology and geometry

Harms et al. (1975, p. 87) provided an early description of HCS (Fig. 6-11A) that has remained fundamentalover the years (cf. Harms et al., 1982). They pointed to four essential characteristics of individual cross-strata setsas being: (1) low-angle (generally less than 10° but up to 15°), erosional bounding surfaces; (2) internal laminaethat are approximately parallel to the lower bounding surface; (3) individual internal laminae that vary systematicallyin thickness laterally and their angle of dip diminishes regularly; (4) dip directions of internal laminae and scouredsurfaces are scattered (i.e., dipping with equal frequency in all directions). They also postulated that thestratification was due to deposition on a scoured bed surface characterized by low hummocks (bed highs) andswales (bed lows) with a spacing of one to a few metres and with a total relief of between 10 and 50 cm. Hence,the form of the internal stratification was one of convex-upward hummocky laminae and concave-upward swaleylaminae, essentially draped over the hummock and swale topography of the basal scoured surface (Fig. 6-11B).

Following Campbell (1966), Dott and Bourgeois (1982) employed a hierarchy of surfaces (Fig. 6-11B) toprovide a careful description of HCS based on their observations. Here we employ their descriptive terms but detailsalso come from other sources (Bourgeois, 1980; Hunter and Clifton, 1982; Brenchley, 1985).

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First-order surfaces are surfaces of lithic change in discrete HCS beds (discussed below) and may boundHCS cosets or beds containing a sequence of various structures. The basal surface is commonly nearly flat anderosional, although local relief, up to several tens of centimetres may occur due to the presence of tool marks(scratches, grooves, prod-marks), gutter casts and/or rare flutes. This surface may be mantled with a lag of coarsedebris of inorganic (intraclasts and/or extraclasts) or biogenic origin (e.g., shell or bone material). In some instances,the upper surface is deeply-scoured with a hummocky appearance whereas in many other cases this surface isrippled. Cosets of HCS range from decimetres to several metres in thickness, although the thickest cosets mayactually consist of several amalgamated beds.

Second-order surfaces are erosional surfaces within HCS cosets and are normally responsible for the formof the stratification. They cut third-order surfaces but are contained by the first-order surfaces and therefore boundHCS sets (Fig. 6-11B). These surfaces commonly form the distinctly “hummocky” surfaces in HCS that arecharacterized by laterally alternating synforms (swales) and antiforms (hummocks), although the antiforms aregenerally less common than the synforms. The relief on second-order surfaces ranges from several centimetresto approximately 50 cm (the same relief reported for hummocks and swales) with rare, extreme dip angles of up to35°. HCS sets range from several centimetres up to two metres in thickness although the latter extremes are probablythe result of the amalgamation of beds. The erosional character of these surfaces may be obvious where relativelysharp, angular discordances occur between second and third-order surfaces or may be subtle where third-ordersurfaces are nearly tangential with second-order surfaces. The angular relationship commonly varies laterally,giving second-order surfaces the appearance of changing from discordant to concordant surfaces along anindividual bed. The visibility of second-order surfaces is due largely to their angular relationship with underlyingstrata. A change in grain size is typically not evident across second-order surfaces although they may be mantledby dispersed shale and/or siderite rip-up clasts, pebbles and shell debris. Such surfaces, exhumed in outcrop,typically display the hummock and swale topography described above. Most commonly, the plan form ofhummocks and swales is circular, although elongate forms have also been reported (e.g., Handford, 1986). Exhumedsecond-order surfaces may display forms of parting lineation including parting-step lineation (McBride and Yeakle,1963) and current lineation (Allen, 1964).

Third-order surfaces bound individual laminae within HCS sets and account for many of the diagnosticcharacteristics of this structure although their visibility may depend on such fortuitous factors such as the degreeof weathering of the outcrop and cementation. Third-order surfaces are nearly concordant with underlying second-order surfaces which normally determine their overall geometry of internal laminae. Angles of dip are typicallyhighest directly above erosional second-order surfaces but decrease upwards. Third-order surfaces are commonlymantled by mica, clay or comminuted plant debris (in many post-Silurian examples). Laminae defined by third-ordersurfaces tend to pinch and swell laterally and are most commonly thickest within swales, thinning upwards overhummocks. Individual laminae may or may not display internal grading, depending largely on the sorting of sand-size particles; well-sorted sand may not have a sufficiently wide range of grain sizes available to develop visiblegrading. For example, Bourgeois’ (1980) observations of the Upper Cretaceous Cape Sebastian Sandstone ofsouthwest Oregon reported centimetre-scale internal laminae in which grading was not detectable. However,Hunter and Clifton (1982), also working on the Cape Sebastian Sandstone, noted that under certain conditions,light/dark couplets that comprise the internal laminae bore characteristics that suggested that they were normallygraded. Like second-order surfaces, exhumed third-order surfaces may display various forms of parting lineation.

The scour and drape form of HCS is the most common variety of this structure although other forms havealso been recognized, including vertical accretionary forms and migrating forms (Fig. 6-16). Several workers (e.g.,Hunter and Clifton, 1982; Bourgeois, 1983; Brenchley, 1985; Allen and Underhill, 1989) described verticalaccretionary HCS in which internal laminae thickened over hummocks rather than swales. As such, the hummocksappeared to have grown by accretion rather than formed by erosion of second-order surfaces. However, this“accretionary” HCS was thought to be relatively rare. A variant of this form of HCS displays internal laminae thatparallel the hummock and swale morphology of second order surfaces (e.g., Allen and Underhill, 1989). Anothertype of HCS, described by Nöttvedt and Kreisa (1987) and Arnott and Southard (1990), is characterized by low-angle cross-strata sets (generally >5 cm thick) filling shallow scours (swales) and which have a preferred, unimodaldip direction; hummocks are generally rare to absent. This latter structure has been termed “low angle trough cross-

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stratification” by Nöttvedt and Kreisa (1989) and “anisotropic” HCS by Arnott and Southard (1990) in contrastto the more common isotropic HCS.

The adherence by later workers to the essentials of the definition of HCS given by Harms et al., 1975) abovehas been a matter for some debate. For example, Brenchley (1985) reported slopes on hummocky surfaces up to35° and spacings as small as the spacing of wave ripples (i.e., centimetres). These deviations from the originaldefinition have led some to suggest that these smaller forms (commonly termed micro-HCS) are not true HCS asHarms et al. (1975) had defined it. For example, Duke (1987, p. 345) argued that HCS-like stratification with hummockspacings below the “1 m lower limit assigned to HCS by Harms et al. (1975)” is not true HCS. However, the firstexperimentally-produced analogs of HCS identified by Harms et al. (1982) had spacings on the order of a coupleof decimetres (and Harms et al., 1982, point out the discrepancy with HCS observed in the field). The scale of HCSmay or may not be a limiting factor in defining HCS but may represent a natural variation in scale that reflects thebreadth of conditions over which this structure may form. Sherman and Greenwood (1989; p. 985) emphaticallystate that “there is no apparent physical rationale for the 1 m lower limit on hummocky cross-stratificationwavelength” and Campbell (1966) suggested that hummocks may occur with wavelengths of 0.1 to 10 m.Alternatively, many of the examples of HCS that differ significantly from the form defined above may representsimilar stratification styles that formed by different processes. The breadth of variation in form of HCS that hasgrown in the literature may be partly responsible for the similar proliferation of ideas regarding its mode of formationthat is discussed below.

HCS associations

With the onset of widespread recognition of HCS, several studies reported its occurrence in particularassociations with other structures (e.g., Hamblin and Walker, 1979; Dott and Bourgeois, 1982; Brenchley, 1985).The most common occurrences of HCS in the ancient record can be classified into two such associations: (1) discretesandstone beds interbedded with mudstones (commonly termed HCS storm beds), and (2) amalgamated sandstones;however, specific associations vary widely in nature (cf. Dott and Bourgeois, 1983).

Discrete HCS sandstones

Dott and Bourgeois (1982) were the first to propose an “ideal sequence” or model showing structures that

Forms of HCS in shallow-marine sandstones

Sco

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Figure 6-16. Schematic illustration showing the various forms of hummocky cross-stratification. Note that the unidirec-tional component of the flow forming anisotropic HCS is from right to left. See text for discussion.

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are preferentially associated in outcrop within sandstone beds containing HCS; an ideal sequence that began alineage of such sequences for HCS sandstones. This sequence consisted of sharp-based sandstone interbeddedwith bioturbated mudstone; the basal-scour surface includes sole marks and is mantled by a lag of coarse debrisoverlain by an interval of hummocky cross-stratification passing upward into flat lamination and ultimately to cross-laminae associated with symmetrical ripple forms that cap the sandstones. A similar sequence of associations wasproposed by Walker et al. (1983) which differed in detail from that proposed by Dott and Bourgeois (1982; 1983)by the occurrence of a basal parallel (horizontal to sub-horizontal) laminated interval directly overlying the basalerosional surface. The evolution of the model continued as more observations were made and data collected. Forexample, figure 6-17 (Leckie and Krystinik, 1989) shows a recent version of the early ideal sequences and containsconsiderably more information and more variability than the earlier sequences; the new information includes theoccurrence of parting lineation on surfaces within HCS, paleocurrent relationships and a range of ripple typescapping the beds, from purely wave-formed ripples through to purely current ripples. The trend of the partinglineation is generally orthogonal to wave-ripple crests and sub-parallel to sole marks at the bases of hummockybeds (e.g., Brenchley, 1985; Leckie and Krystinik, 1989). Rarely, a polymodal trend of parting lineation has beenobserved on second or third-order surfaces (D.A. Leckie and L.F. Krystinik, unpublished observations). Cappingwave-ripples are typically straight crested with bifurcating patterns, although irregular forms are not uncommon,including: polygonal, ladderback and box patterns (all forms of interference ripples). In addition, Leckie andKrystinik (1989) include directional relationships between structures in the HCS beds with regional shoreline andpaleoslope (Fig. 6-17). Specifically, they showed that directional structures such as sole marks and parting lineationindicate paleoflows directed offshore, orthogonal to regional shoreline-trend indicators. Similarly, capping rippleshave crests aligned approximately parallel to regional shoreline and the internal cross-laminae, when present,indicate migration offshore. Such relationships had been suggested earlier on the basis of local studies (e.g.,Hamblin and Walker, 1979; Brenchley 1985, Rosenthal and Walker, 1987) but data provided by Leckie and Krystinik(1989) suggested that the directional associations may be the norm for discrete HCS sandstones.

Amalgamated HCS Sandstones.

This association of HCS is characterized by thick (up to several tens of metres) sandstones and differs fromthe other association by the lack of mudstones (except as local lenses) and the absence of a preferred sequenceof structures. Amalgamated HCS commonly occur above the discrete HCS beds in regressive shorelinesuccessions (e.g., Hamblin and Walker, 1979; Leckie and Walker, 1982) and is representative of sedimentation inthe lower shoreface. First-order surfaces may be recognized within amalgamated sandstone beds by the presenceof a lag or where they overlie intensely bioturbated horizons, discontinuous mudstone beds or concentrations ofmica and fine plant debris.

Swaley cross-stratification, as originally defined by Leckie and Walker (1982), is not the amalgamated HCSas described here, although there is a growing tendency amongst some authors to state this. For example, Dottand Bourgeois (1983), McCrory and Walker (1986) and Plint and Walker (1987) suggest that the swaley cross-stratification is typical of amalgamated sandstone beds. Duke (1985, p. 171), however, specifically stated thatswaley cross-stratified sandstones do not show evidence of amalgamation. In a vertical, progradational successiondiscrete HCS is overlain by amalgamated HCS which, in turn, is overlain by SCS.

The HCS Debate

When Harms et al. (1975) first introduced HCS they suggested that the structure was the product of theinteraction of waves with a sandy substrate during powerful storms (i.e., the waves that produced HCS wereparticularly large and powerful). Emplacment of sand by storms is widely accepted because discrete HCS bed,encased in shale, must represent extreme sediment transport events that introduced sand into a marine setting thatreceived only deposition of mud during periods of “normal sedimentation”; storms are known to provide such amechanism of sand emplacment. The specific depositional setting of the HCS storm beds is thought to be at depthsabove 200 m (the approximate maximum depth of the continental shelf) but blow the depth where normal, fairweatherwaves will affect a sediment substrate (i.e., below effective fairweather wave base). This setting is suggested bythe normal mud deposition and by the trace fossils and body fossils associated with HCS storm beds. In supportof a storm origin are estimates of the recurrence intervals of emplacement of the sands forming discrete storm beds.

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Table 6-2 shows that independent estimates suggest that HCS storm beds are emplaced only every few thousandyears. Hence the intensity of storms that transported the sand out onto the shelf and formed the sequence ofstructures within the deposits were likely of an intensity not yet recorded. However, as more became known aboutHCS the role of waves in its formation became less certain. Particularly, the presence of unidirectional paleocurrentindicators (such as sole marks on the bases of HCS storm beds), along with the recognition of anisotropic HCS,suggested that unidirectional currents may play a role in forming this structure. The importance of unidirectionalcurrents in the emplacement of HCS storm beds in settings that normally received mud is obvious: the sand istransported offshore, from beach and near-shore environments where it dominates during fairweather conditions,and such transport requires a directed current (i.e., a unidirectional current). Furthermore, such unidirectionalcurrents are well known to modern oceanographers who have measured their intensity during historically moderateto large storms.

In light of the recognition of the importance of unidirectional currents in emplacing HCS storm beds severalnew mechanisms of HCS formation were proposed, stressing the importance of unidirectional currents. Swift etal. (1983) reported HCS on modern shelves in 15 to 40 m water depth. Their examples of HCS collected from wave-modified dunes that formed under combined flows, with a powerful unidirectional component, generated by storms.Box cores taken from the bedforms displayed wedge-shaped sets of heavy mineral-rich laminae in fine to very fine-grained sands. Swift et al. (1983) explained the wedge-shaped sets as curved lamina intersections and likened thestructure to HCS. Hence, the HCS was attributed to combined flows, and specifically to wave modification of whatwould have otherwise been dunes under unidirectional flows. Elsewhere, Greenwood and Sherman (1986)suggested that, in a lacustrine setting, a unidirectional flow (in their case, a longshore current) was crucial to theformation of HCS. They argued that without the combined-flow component, purely oscillatory waves would onlyproduce flat-bed conditions. Similarly, Allen (1985) argued convincingly on theoretical grounds that oscillatorycurrents alone could not form hummocky bedforms of metre-scale wavelength, the development of which requireda unidirectional current.

The above debate progressed on the basis of inference from the ancient record and from observations in

Figure 6-17. Ideal sequence of structures found in HCS storm beds. See text for discussion. From Leckie and Krystinik (1989).

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the modern , shallow-marine environments where no unequivocal HCS could be identified. Over 1990-91 the resultsof two different approaches to the study of HCS were reported that added more factual knowledge of HCS thatpointed to a return to the original ideas on HCS formation under waves. The new data included experimental studiesand studies of grain fabric in HCS sandstones.

Experimental Evidence

Southard et al. (1990) conducted experiments in a wave duct that produced three-dimensional bedforms(large, 3-D vortex ripples) that behaved in such a way under the purely oscillatory currents that they would producea form of stratification that very closely resembled HCS observed in the ancient record (Fig. 6-18). This experimentalevidence showed that purely oscillatory flows could form HCS but did not rule out the possibility that combinedflows could also produce this structure. However, the combined-flow experiments of Arnott and Southard (1990)appeared to eliminate combined flows as having an important role in forming HCS. In their experiments, symmetricalbedforms could only be produced under purely oscillatory flows or combined flows with a negligible unidirectionalcomponent (Fig. 6-13). As soon as the velocity of the unidirectional component exceeded a few percent of themaximum orbital velocity the bedforms became asymmetrical and the stratification that they produced would appearas anisotropic HCS. These experiments suggested that combined flows with strong unidirectional currents couldnot form HCS, in contrast to the result of Allen’s theoretical analysis and the suggestions of oceanographers.

Evidence based on grain fabric

A detailed study of grain fabric (Fig. 6-19) was reported by Cheel (1991) based on samples oriented withrespect to sole marks on the base of discrete HCS sandstone beds interbedded with shale. This study showedthat particle a-axes, measured in plan view (Fig. 6-19A) varied widely but displayed modes oriented approximatelynormal to sole marks and parallel to the associated ripple crest. This suggests that a-axis alignment of grains inHCS develops by rolling (a-axes transverse to the oscillatory current) and the wide variation in a-axes orientationpoints to deposition under a complex array of surface gravity waves with a mode aligned parallel to the shoreline.In vertical section through HCS sandstones imbrication of grains varied about a mean of 0°, parallel to visiblelamination (Fig. 6-19B). In some cases, this variation in imbrication was markedly cyclic about the 0° mean. Thispattern of imbrication was interpreted in terms of the action of symmetrically oscillating currents during the

Table 6-2. Estimates of recurrence intervals between events (storms?) that emplaced HCS storm beds.

Age Recurrence Interval Author

Kimmeridgian 3,200 - 4,000 years Hamblin and Walker (1979)Walker (1985

Devonian 400 - 2,000 years Goldring and Langenstrassen (1979)

Ordovician 10,000 - 15,000 years Brenchley et al. (1979)

Triassic 2,500 - 5,000 years or Aigner, 19825,000 - 10,000 years

Ordovician 1,200 - 3,100 years Kreisa, 1981

Figure 6-18. HCS simulated from large 3-D vortex ripples. Total length of bed shown in 2.15 m. From Southard et al.(1990).

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L

θ

0

2

1

DIS

TA

NC

E A

BO

VE

DA

TU

M (

MM

)

-90 90ANGLE ( )

0

= Lθ

FLOW

DIS

TA

NC

E A

BO

VE

DA

TU

M (

MM

)

B

C

341

˚

ANGLE ( )-90 900

2

1

0

˚

566

A REGIONALSHORELINE

C

A

B

PROD MARKS

294

D

Figure 6-19. Grain fabric in a hummocky cross-stratified sandstone. A. Apparent grain long axes as seen in plan view on surfacescut parallel to bedding. B and C. Apparent grain long axes seen in vertical section on surfaces cut perpendicular to beddingand parallel from inferred flow direction. Note that the relative positions of A and B are shown in the block diagram.

formation of isotropic HCS. In contrast, in the basal parallel-laminated interval of an HCS bed, the mean particleimbrication was approximately 13° into the flow direction (based on the sole marks) and varied quasi-cyclically aboutthat mean (Fig. 6-19C). The interpretation of this pattern of variation in fabric suggested that an offshore-directedunidirectional current was active during deposition of the horizontally-laminated portion of HCS storm beds(producing the onshore imbrication). However, when the HCS formed this unidirectional component had eitherstopped or had become too weak to influence grain fabric. Thus, in the HCS storm beds it appeared that HCS formsin response to oscillatory flows, the same conclusion that arose from the experimental evidence.

Origin of HCS?

Given the above interpretation of HCS storm beds we must explain the different forms of HCS that have beenobserved (Fig. 6-20). The HCS storm beds displaying isotropic HCS form as sediment is delivered onto the shelfby offshore-directed combined flows generated by storms; combined flows with powerful oscillatory componentsdue to large gravity waves and offshore (or offshore-oblique) unidirectional components. Such currents are well-known to modern oceanographers, although their directional relationship to the shoreline is more complex thandescribed here (see Duke 1990 for a full discussion). While the unidirectional component of the current is activenot only is the sediment transported offshore but erosion of the substrate occurs, forming the basal solemarksoriented onshore-offshore and any directional solemarks (such as flutes and prod marks) are directed offshore.

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With the onset of sand deposition conditions are initially on a flat bed (which has a wide stability field undercombined flows; see Fig. 6-14) but this bed state is replaced by large, 3-D vortex ripples as the unidirectional currentwanes and only a powerful oscillatory current continues. Deposition under this oscillatory flow forms isotropicHCS. As the oscillatory flow wanes small, 2-D vortex ripples are formed, capping the sandstone. Under fairweatherconditions, following the storm, muds are deposited, encasing the storm-deposited sandstone bed.

The above scenario accounts for much HCS associated with HCS storm beds but it does not explain thevariety of forms of HCS shown in figure 6-16. Certainly, the isotropic forms are likely formed by oscillatory currents.However, the interaction of these currents with a sandy substrate must differ in detail to produce scour and drapeversus vertical accetionary HCS. In the case of the scour and drape form of HCS the fact that internal laminae drapeand diminish the hummocky relief suggests that a hummocky bedform is not stable under conditions that form thestructure but are only stable during periods of intense flow that causes erosion of the hummocky surface. Sucherosion during a storm might occur due to the temporary development of constructive waves on the water surface.Swift et al. (1983) described the generation of thick clouds of sediment that rose off the bottom during a storm withthe passage of groups of exceptionally high waves formed by constructive interference. The formation of theseclouds must involve the local addition of sediment into suspension by erosion that might form hummocky second-order surfaces. Between periods of wave construction the “normal storm waves” would act as sediment continuedto deposit, causing the temporal variation in flow strength that results in the formation of internal laminae. In thisscenario the hummocky surface is not due to the generation of a stable bedform but is inherited from the form ofan erosional surface, a surface that might mimic the morphology of the stable bedform under the same conditionsbut with net deposition on the bed. Subsequent deposition of sediment onto the hummocky erosional surface actsto bring the topography into equilibrium with the normal storm-wave conditions (and this equilibrium bed appearsto be more-or-less flat). Hence, the currents that produce the hummocky form are associated with erosion, duringthe most intense conditions on the bed.

SUBSTRATE

BED RESPONSE

BED STATE

CURRENT

EROSION

WANING OSCILLATORY FLOWCOMBINED FLOW

FLAT BED

HUMMOCKY BED 2-D WAVERIPPLES

IRREGULAR SCOUR ANISOTROPIC ISOTROPIC

DEPOSITION

NO BED-FORMS

OF

FS

HO

RE

ON

SH

OR

E

0

TIME

SHORE-NORMALFLOW SPEEDNEAR THE BED

INSTANTANEOUSTIME-AVERAGED

cm -

dm

Figure 6-20. Schematic illustration of the temporal variation in currents involved in the formation of HCS storm beds. Seetext for discussion. After Cheel (1991), Duke, Arnott and Cheel (1991) and Cheel and Leckie (1992).

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Vertical accretion forms (Fig. 6-16), characterized by thickest laminae within hummocks, appears to involvethe growth of a depositional hummocky topography. This type of HCS might represent the product of currents,possibly produced by sustained constructional waves that are less transient than the case of scour and drape.Under conditions of rapid deposition, such sustained currents, capable of building a stationary hummocky bedformthat is in equilibrium with the prolonged, oscillatory currents generated by the constructional waves, would formvertical accretionary sets. Hummocky bedforms that would produce such stratification were produced under purelyoscillatory flows and strongly oscillatory combined flows by Arnott and Southard (1990). In this case, a truebedform is constructed under conditions of net aggradation.

The low angle migratory forms (Fig. 6-16) may represent similar bedforms that form accretionary HCS butwhich migrate over the substrate, preferentially preserving internal laminae that dip in the direction of migration(Nöttvedt and Kreisa, 1982). Arnott and Southard (1990) produced stationary and migrating bedforms experimen-tally in a wave duct. The stationary forms were stable under oscillatory flows and strongly oscillatory-dominantcombined flows. However, migrating forms developed when the velocity of the unidirectional component exceededa few percent of the orbital velocity of the oscillatory component. Hence, anisotropic HCS may be interpreted toindicate the influence of a unidirectional component of a combined flow under conditions that would otherwise formisotropic HCS.

Conclusion

The debate over the origin of HCS continues and has become more perplexing over the same period thatnew evidence for a wave-induced origin. Work by Rust and Gibling (1989) and Prave and Duke (1990) providedexamples of cross-stratification that appears like HCS but was produced by three-dimensional in-phase waves. Nowwe have an HCS-look-alike. The paleoenvironmental interpretation of each structure is radically different; HCSformed by in-phase waves is definitely indicative of unidirectional flows whereas the shallow marine forms of HCSappear to have been largely influenced by waves. The most promising approach to distinguishing the two formsis through detailed studies of grain fabric and this research is on-going and the general debate continues.